1. Field of the Invention
The present invention relates to a continuously variable V-belt transmission including hydrodynamic transmission unit with a lock-up means, and more particularly to a lock-up control for the hydrodynamic transmission unit.
2. Description of the Prior Art
A conventional continuously variable V-belt transmission is disclosed in laid-open Japanese patent application No. Sho 55-60755 which corresponds to U.S. Pat. No. 4,274,520 and is illustrated in FIG. 1. This continuously variable transmission comprises a fluid coupling 201 with a lock-up device in the form of a multiple disc clutch disposed within the fluid coupling, a drive pulley 205 including a fixed cone conical disc 203 and a movable conical disc 204, and a V-belt 206 interconnecting a driven pulley (not illustrated) and drive pulley 205, wherein output from a turbine runner 201a of the fluid coupling 201 is delivered to the drive pulley 205 and then to the driven pulley via the V-belt 206. For accomplishing a continuously variable ratio change, the radius of the diameter of the drive pulley 205 contacting with the V-belt 206 is changed by urging the movable conical disc 204 in an axial direction. The movement of the movable conical disc 204 is accomplished by the oil pressure acting within the pulley cylinder chamber 207 and the amount of movement is determined by the magnitude of the oil pressure therein. When the lock-up device is actuated by applying oil pressure to the cylinder chamber 208 to press the piston 209, the pump impeller 201b of the fluid coupling 201 is mechanically connected with the drive pulley 205, thus allowing the fluid coupling to stop operating as a fluid coupling. The pulley cylinder chamber 207 of the drive pulley 205 and a cylinder chamber 208 of the lock-up device 202 are communicated with each other so that they receive common oil pressure from an oil conduit 210. Accordingly, when the oil pressure within the oil conduit 210 is low, a transmission ratio shifts toward a low side (viz., a side wherein a reduction ratio is great) and the lock-up device 202 is left unactuated so that the fluid coupling resumes its function as a fluid coupling. Conversely, when the oil pressure within the oil conduit 210 increases, the transmission ratio shifts to a high side (viz., a side wherein the reduction ratio is small) and the lock-up device is actuated to allow the fluid coupling 201 to operate in a lock-up state (engaged state).
However, since, in such a conventional continuously variable transmission, the oil pressure supplied to the pulley cylinder chamber 207 of the drive pulley 205 is the same as that supplied to the cylinder chamber 208 of the lock-up device 202 and this oil pressure is variable to provide a variable reduction ratio, there is a problem that since, when the oil pressure is low, the oil pressure within the cylinder chamber 208 of the lock-up device is accordingly low, the multiple disc clutch may be kept in half engaged state, accelerating the rate of wear of the clutch discs or causing a damage on the discs, while, when the oil pressure is high (viz., when the reduction ratio is small), a great force is impressed upon the multiple friction discs, thus breaking them off.